Pressure relief valves are commonly used to protect flow lines and associated equipment from overpressure events. Such valves normally include an inlet which is connected to the flow line, an outlet which is connected to a vent line and a valve element which is movable between a closed position in which the outlet is isolated from the inlet and an open position in which the outlet is open to the inlet. In certain types of pressure relief valves the valve element is maintained in its normally closed position by fluid in a pressure cylinder which acts on a piston that is connected to the valve element. In order to maintain the valve element closed against normal operating pressure in the flow line, the fluid in the pressure cylinder is pressurized to a predetermined set point. Then, when the pressure in the flow line exceeds a predetermined maximum value, the force imbalance created by the overpressure will force the valve element open and allow the fluid in the flow line to flow through the outlet and into the vent line.
In the well servicing industry, well operators sometimes require the use of “full opening” pressure relief valves. These types of pressure relief valves employ a large pressure cylinder which is filled with a compressible gas. The pressure in the pressure cylinder is limited by the available gas storage supply and therefore usually requires that the area of the piston be six to eight times larger than the area of the valve element which seals the inlet. In addition, the gas pressure in the cylinder must be maintained at a predetermined set point pressure to allow the valve element to open at the desired maximum line pressure.
However, full open pressure relief valves have several shortcomings. First, they normally require additional components such as a control panel, a high pressure gas bottle and multiple hoses. Consequently, these valves are usually complex, large and difficult to set up. In addition, the gas cylinder, although pre-charged, will still exhibit a spring rate that requires the line pressure to exceed the desired maximum by some amount before the valve fully opens. Thus, even if the valve comprises quick exhaust ports, the large volume of gas required to be exhausted from the pressure cylinder will delay full opening. Furthermore, the pressure in the gas cylinder will often change with changes in ambient temperature at the job site. For example, a 40 degree change in temperature may increase the set point by about 10%, which has been deemed unacceptable by many well operators.